Eukaryote DNA repair systems are important in repair of radiation and chemical damage, in mutation induction, in recombination, and probably in cancer induction and in normal DNA and RNA synthesis. Because the nature of post-replication-repair of DNA has been particularly difficult to study in eukaryotes, we propose to study this system in the fungus Neurospora crassa which is amenable to rapid genetic and biochemical analysis and in which a number of well characterized repair-defective mutants are available. Methods are being developed to extract long, highly radioactive DNA from Neurospora directly on top of alkaline-sucrose-gradients. Changes in the molecular weight of newly made DNA in known excision-repair defective and presumed post-replication-repair defective cells will be monitored by alkaline sedimentation centrifugation after ultraviolet light exposure followed by various recovery periods with and without the presence of caffeine. Evidence for the involvement of recombination dependent steps will be obtained both from the pattern of recombination defects in the mutants examined and from an examination of the transfer of dimers from old to new DNA, using the T4 endonuclease assay. Isolation of mutants able to incorporate deoxythymidylate monophosphate directly into DNA will also be attempted.